The bone marrow serves as an ideal source for testing and correlating the surface histochemistry and morphological aspects of cell differentiation since all phases in the growth and maturation of several structurally and functionally independent cell lines exist concurrently in the marrow, from committed stem cells to fully differentiated leukocytes and erythrocytes. Transmission electron microscopy in concert with histochemical probes for surface anionic groups, specific carbohydrate residues of glycoproteins and gangliosides and several polypeptide hormone receptor sites will be studied in the marrow of guinea pig and man. A histochemical profile of the surface characteristics of individual cell types and the types of chemical modulations occuring during the maturation of each independent cell line will be obtained. The ultrastructural histochemical methods used yield particulate deposits allowing quantification of the extent of surface labeling for each cell. Data will be analyzed statistically to determine if significant differences exist in surface reactivity between groups of cells and during their maturation. This histochemical approach has the advantage over biochemical and radioassay methods since individual cells can be analyzed in the heterogeneous cell populations inherent in the marrow; the latter methods ideally require homogeneous cell populations. Since surface factors controlling the growth, proliferation and maturation of the individual cell lines in the marrow are generally unknown, data derived in this study will shed considerable light on the chemical properties of cell surfaces during the growth and differentiation of hemic cells.